# std::thread::join

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< cpp‎ | thread‎ | thread

void join(); |  | (since C++11)

Blocks the current thread until the thread identified by *this finishes its execution.

The completion of the thread identified by *this synchronizes with the corresponding successful return from `join()`.

No synchronization is performed on *this itself. Concurrently calling join() on the same thread object from multiple threads constitutes a data race that results in undefined behavior.

### Parameters

(none)

### Return value

(none)

### Postconditions

joinable() is false.

### Exceptions

std::system_error if an error occurs.

### Error conditions

- resource_deadlock_would_occur if this->get_id() == std::this_thread::get_id() (deadlock detected).
- no_such_process if the thread is not valid.
- invalid_argument if joinable() is false.

### Example

Run this code

    #include <chrono>
    #include <iostream>
    #include <thread>

    void foo()
    {
        // simulate expensive operation
        std::this_thread::sleep_for(std::chrono::seconds(1));
    }

    void bar()
    {
        // simulate expensive operation
        std::this_thread::sleep_for(std::chrono::seconds(1));
    }

    int main()
    {
        std::cout << "starting first helper...\n";
        std::thread helper1(foo);

        std::cout << "starting second helper...\n";
        std::thread helper2(bar);

        std::cout << "waiting for helpers to finish..." << std::endl;
        helper1.join();
        helper2.join();

        std::cout << "done!\n";
    }

Output:

    starting first helper...
    starting second helper...
    waiting for helpers to finish...
    done!

### References

- C++23 standard (ISO/IEC 14882:2024):

  - 33.4.3.6 Members [thread.thread.member]

- C++20 standard (ISO/IEC 14882:2020):

  - 32.4.2.5 Members [thread.thread.member]

- C++17 standard (ISO/IEC 14882:2017):

  - 33.3.2.5 thread members [thread.thread.member]

- C++14 standard (ISO/IEC 14882:2014):

  - 30.3.1.5 thread members [thread.thread.member]

- C++11 standard (ISO/IEC 14882:2011):

  - 30.3.1.5 thread members [thread.thread.member]

### See also

detach | permits the thread to execute independently from the thread handle (public member function)
joinable | checks whether the thread is joinable, i.e. potentially running in parallel context (public member function)
C documentation for thrd_join

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